(1) Field of the Invention
The present invention relates to a membrane filter of a bismaleimide polymer having an excellent organic solvent resistance and a method for the formation thereof. More particularly, the present invention relates to a material for a membrane filter used for separating floating dusts, colloids, proteins and the like by macrofiltration or ultrafiltration, a method for forming a hollow yarn and a plain film from this material, and a preferred process for the preparation of a bismaleimide resin used for this membrane filter material.
(2) Description of the Related Art
Ultrafiltration membranes and selective permeation membranes are frequently used at the present because they can separate colloids, proteins, microorganisms and the like from dispersion media at high efficiencies. Furthermore, these filtration membranes are utilized for the waste water treatment in plants, buildings and the like, and they are also used in the fields of the food industry, medical industry and agriculture. In general, the membrane filter is formed into a plain membrane or a hollow fiber. The hollow fiber is widely used as a dialysis module for separating colloids, proteins, microorganisms and the like from a dispersion medium at a high frequency in the medicinal field. An aqueous solution has heretofore been used as the dispersion medium (the solution used for the module or the like), but with recent diversification of the technique, organic solvents are used as the dispersion medium. Furthermore, organic solvents are often used for regeneration and washing of hollow yarns. Therefore, development of a membrane filter having an excellent organic solvent resistance is desired.
As the material constituting the membrane filter, there have been used various organic polymers such as a polysulfone (Japanese Examined Patent Publication No. 56-54164), a photo-crosslinked polymer (Japanese Unexamined Patent Publication No. 59-136107 and Japanese Unexamined Patent Publication No. 58-84006) and a fluorine polymer (Japanese Unexamined Patent Publication No. 59-166541). Furthermore, there are known membrane filters composed of cellulose acetate, polyacrylonitrile or a polyamide. Moreover, a hollow yarn composed of a polyether-sulfone is used for removing proteins and the like from a dispersion medium at a high efficiency. A membrane filter composed of cellulose acetate or a polyamide is defective in that the heat resistance is poor. A filter membrane composed of a polysulfone has a large permeation amount of water, a high resistance to an acid and an alkali and a good heat resistance, but the organic solvent resistance is poor. A photo-crosslinked polymer has an organic solvent resistance, but the permeation of water or the like is insufficient. A fluorine polymer has an organic solvent resistance, but this polymer is defective in that since there is no appropriate solubilizing agent to be used for formation of a film, it is difficult to prepare a plain membrane and it is impossible to reduce the pore size.
A conventional hollow fiber composed of a polyether-sulfone is defective in that when acetone or an aromatic solvent is used as the dispersion medium, such troubles as damage by dissolution or swelling are caused. A fluorine resin can be mentioned as a membrane material having an organic solvent resistance, but this resin is poor in the solubility and there is not a suitable spinning solvent, and therefore, it is difficult to prepare a hollow yarn.
Under this background, we made investigations and found that a specific bismaleimide resin is suitable as a material of a selective membrane filter. However, a product obtained according to the conventional process for the preparation of a polymer of a bismaleimide compound has a low molecular weight, and some polymers are soluble in cresol and dichloromethane. As another method for the preparation of a bismaleimide resin, there is known a process comprising reacting bismaleimide with piperazine in dichloromethane or N-methylpyrrolidone (see Japanese Unexamined Patent Publication No. 50-142700). However, in such a solvent, gelation is promptly advanced, and the formed bismaleimide resin is not suitably used as a membrane material.